Selection of superior accessions of turnip (Brassica rapa var. rapa L.) based on tuber quality‐related characters

Abstract Turnip (Brassica rapa var. rapa L.) (syn. B. campestris L. ssp. rapifera Sinsk) is an important crop species belonging to the Brassicaceae family. The 185 accessions belonging to this crop were collected from several areas of Toodshak region in Isfahan province, Iran, and their tubers were cultivated under homogeneous conditions in loamy clay soil. The morphological traits of different organs of those accessions were evaluated. Significant variations were detected among the accessions studied based on the traits recorded. Tuber shape showed high variation and included globose, oblong, ovate, obovate, and fusiform. Also, tuber skin color was highly variable, including white, bicolor white–violet, light violet, and dark violet. Tuber weight ranged from 1.56 to 35.70 g, while total soluble solids (TSS) of tuber flesh ranged from 7.00 to 11.80%. Principal component analysis (PCA) showed that 18 components were extracted by explaining 74.88% of total variance. The dendrogram obtained based on all the characters measured clustered the accessions into two major clusters. Sixteen accessions were placed into the first cluster, while the remaining accessions were placed into the second cluster which was divided into six subclusters. High level of morphological variabilities was observed among the accessions, which is applicable and useful for B. rapa var. rapa breeding programs. Based on the commercial and quality traits, 17 accessions could be selected for direct cultivation. Also, the promising accessions identified here can be utilized directly in breeding programs for genetic enhancement of this crop.

is consisted of the swollen hypocotyl and root (Kubo et al., 2019). It is generally grown as a field vegetable or home garden crop (Abbasi et al., 2011). Turnip is greatly consumed in Europe, Asia, America, and North Africa due to its availability in local markets, cheapness, and consumer preference (Haliloglu et al., 2012;Li et al., 2018).
Genetic diversity is a prerequisite in crop improvement programs. It aids in choosing parental materials to get maximum recombination in hybridization process (Arunachalam, 1981).
Diversity in germplasm is fundamental in selecting superior accessions having advantageous traits for utilization in hybridization in developing varieties with increased yield (Joshi & Dhawan, 1966) and wider adaptation, desirable quality pest, and disease resistance (Nevo et al., 1982). Besides genetic variability in the germplasm, the effectiveness of selection for a particular trait depends on the degree of association that exists between different traits.
Analysis of variability among the traits and the association of a particular character in relation to other traits contributing to yield of a crop would be of great importance in planning a successful breeding program (Mary & Gopalan, 2006). The main aim of plant breeders is to study the genetic variability of new accessions of important crop species and to screen best accessions for efficient utilization or varieties development. Estimation of various qualitative and quantitative traits provides a clear picture to improve diverse accessions. Among these characters, the yield parameter plays a vital role for new variety development (Ali et al., 2013;Azam et al., 2013).
Morphological characterization continues to be the first step in the description and classification of germplasm (Marjanovic-Jeromela et al., 2011). Cluster and principal component analyses are useful techniques to be used for the classification of different biological populations at genotypic level and to evaluate comparative influence of various components to the total divergence both at intra-and intercluster levels. In plant breeding programs, several characters are simultaneously considered making it feasible to approximate the genetic divergence using multivariate techniques. Multivariate analyses have equivalent usefulness to create the most proper cross combinations (Zahan et al., 2008).
Collections of local landraces/germplasm from diverse locations have great value from breeding point of view as containing many hidden desirable genes. For a successful plant breeding program, it is necessary that germplasm have diversity, reproducibility, and easily availability to be used in the development of new cultivars (Naushad et al., 2015;Zada et al., 2013). The present study attempted to study agro-morphological-based variation among different local accessions of B. rapa var. rapa for economically important quantitative traits.

| Plant material
The 185 accessions belonging to B. rapa var. rapa were collected from several areas of Toodshak region in Isfahan province, Iran. The tubers of the collected accessions were cultivated in Saveh region, Markazi province, Iran, under homogeneous conditions in loamy clay soil.

| The characters evaluated
In total, 63 morphological characters were used to study phenotypic diversity (Table 1). A digital caliper was used to measure dimensionsrelated characters of leaf, flower, tuber, silique, and seed. Also, tuber weight, silique weight, and 1000-seed weight were measured with an electronic balance with 0.01 g precision. Furthermore, the qualitative characters were estimated based on rating and coding (

| Statistical analysis
Analysis of variance (ANOVA) was performed to evaluate the variation among accessions based on the traits measured using SAS software (SAS Institute, 1990). Simple correlations between traits were determined using Pearson correlation coefficients (SPSS Inc., Norusis, 1998). Principal component analysis (PCA) was used to investigate the relationship between accessions and determine the main traits effective in genotype segregation using SPSS software.
Hierarchical cluster analysis (HCA) was performed using Ward's method and Euclidean coefficient using PAST software (Hammer et al., 2001). The first and second principal components (PC1/PC2) were used to create a scatter plot with PAST software.

| RE SULTS AND D ISCUSS I ON
Significant variations were detected among the accessions studied based on the traits recorded as revealed with ANOVA. Forty-nine of 63 characters measured showed CV more than 20.00%, 20 characters showed the CV more than 50.00%, and three characters exhibited the CV more than 100.00%, indicating high diversity among the accessions. Tuber flesh bitterness had the highest CV (344.17%), followed by leaf lamina blistering (262.27%) and vivipary (228.13%).

TA B L E 3 (Continued)
and total leaf number (0.84). The remaining characters loaded significantly in the rest components (PC4-PC18) and explained less variability.
The scatter plot created based on the PC1 and PC2 (Figure 3) showed that the accessions with close proximity were more similar in terms of effective traits in PC1 and PC2 and were placed in the same group. The scatter plot showed that residuals of the majority of accessions bounce randomly around 0.00 line forming the horizontal band. This suggests that the variances of the error terms are equal and the relationship among the accessions is linear. However, 10 outliers were observed among the accessions evaluated, which might be due to their extreme values for particular traits.
The dendrogram obtained based on all the characters measured clustered the accessions into two major clusters (Figure 4).
Sixteen accessions were placed into the first cluster. The rest of the accessions were placed into the second cluster, which was divided into six subclusters, indicating high variability among the accessions.
Here, a high level of morphological variabilities was observed among the accessions, which is applicable and useful for B. rapa var.
Agro-morphological-based variation is important to screen best accessions in field experiment. The diverse agro-morphologicalbased accessions are useful for further biochemical and molecular evaluation. Genetic diversity study is used for efficient utilization and for development of improved cultivar/varieties (Jan et al., 2016;Shinwari et al., 2014). Morphological-based screening of different crop species/subspecies is therefore so much important for all plant breeders (Iqbal et al., 2015;Martins et al., 2006). Therefore, proper strategies and planning is needed to evaluate local and exotic germplasm and to screen best accessions among these for both qualitative and quantitative characters (Balkaya & Ergun, 2008). The conservation strategies are important for many reasons as it is best source to conserve threaten/endangered species, for further crop improvement through new morphological techniques, and to develop new breeding cultivars/varieties (Baranger et al., 2004).  , including no. 40, 72, 17, 10, 7, 19, 112, 98, 83, 2, 4, 1, 5, 9, 37, 6, and 8, could be selected for direct cultivation. Also, the promising accessions identified here can be utilized directly in breeding programs for genetic enhancement of this crop.

F I G U R E 3
Scatter plot for the studied B. rapa var. rapa accessions based on PC1/PC2

ACK N OWLED G EM ENT
None.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

R E S E A RCH I N VO LV I N G H U M A N PA RTI CI PA NTS A N D/ O R A N I M A L S
None.

I N FO R M E D CO N S E NT
None.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.